1. Field of the Invention
The present invention relates, in general, to an emergency core cooling systems for pressurized light water reactors (PLWR) and, more particularly, to a direct vessel injection-type pressurized light water reactor (DVI-PLWR), in which an emergency core cooling water (ECC) is directly injected into a downcomer of a reactor vessel through ECC injection pipes, and which is provided with flow converting grooves in the downcomer to convert the flow of the injected ECC, tending to be bypassed to a broken area of a broken cold leg while being carried along in a high-speed lateral flow of steam generated in the downcomer and rushing therewith into the broken cold leg in the case of a cold leg guillotine break (CLGB), such as a double-ended guillotine break (DEGB), into stagnant vortexes, and to allow the stagnant vortexes of the ECC to flow down in the downcomer due to gravity, thus reducing the ratio of emergency core cooling water bypass (ECC bypass), and thereby allowing a large amount of ECC to reach the reactor core by way of the lower section of the downcomer.
2. Description of the Related Art
As shown in FIGS. 1A and 1B, a conventional pressurized light water reactor (PLWR) includes a reactor vessel 100 fabricated with two cylindrical parts, that is, a pressure vessel 5 and a core barrel 10. In the PLWR, the pressure vessel 5 defines an outer vessel of the reactor vessel 100, while the core barrel 10, having a diameter smaller than that of the pressure vessel 5, is concentrically placed in the pressure vessel 5. A reactor core 7 is provided at the center of the core barrel 10 to hold a plurality of nuclear fuel rods (not shown) in the core barrel 10. Due to the diameter difference between the pressure vessel 5 and the core barrel 10, an annular space is defined between the pressure vessel 5 and the core barrel 10 to form a downcomer 20 in the reactor vessel 100. The PLWR also includes a plurality of cold legs 15 and hot legs 25. The cold legs 15 are connected to the pressure vessel 5 and act as passages for cooling water, while the hot legs 25 are connected to the core barrel 10 and guide the cooling water, which is heated to a high temperature while passing through both the downcomer 20 and the reactor core 7 after flowing into the pressure vessel 5 through the cold legs 15, to a steam generator (not shown) of the PLWR.
The PLWR is a nuclear energy system operated with a nuclear fuel, which is a highly radioactive substance, as an energy source, so that the PLWR may cause a terrible accident accompanied by many casualties in the case of an accident at the PLWR, such as a reactor break. Therefore, it is necessary to secure operational safety of the PLWR, and for the operational safety, the PLWR must meet the strict safety standards at each step of design, construction, and operation of the PLWR.
As an example of conventional safety standards established to regulate the PLWR, there are proposed performance and safety standards governing the cooling systems of the PLWR. The above performance and safety standards for the PLWR cooling systems are typically used as the regulatory technical standards in the technical evaluation of the design, construction and operation of the PLWR, which evaluation is performed by an expert regulatory organization, such as the Korean Institute of Nuclear Safety (KINS) before granting permission for the design, construction and operation of the PLWR. The most important item of the performance and safety standards established to regulate the PLWR is an item concerning the large break loss-of-cooling water accident (LBLOCA) caused by a double-ended guillotine break (DEGB) of cold legs. The LBLOCA caused by the DEGB typically results in an increase of the cladding temperature of fuel rods to the highest point expected for cladding temperatures in the case of PLWR safety accidents. In the safety standards for the LBLOCA caused by DEGB, the safety criteria is established to determine whether the highest cladding temperature of the fuel rods in the case of DEGB is maintained at a level lower than the regulatory level, and to determine whether the reactor core is maintained at an effectively cooled state.
In an effort to meet the safety standards for the PLWR, the PLWR is provided with an emergency core cooling water injection pipe (ECC injection pipe) 30 to accomplish an injection of the ECC into the reactor vessel 100 to cool the reactor core 7 in the case of accidents in the PLWR. That is, a plurality of ECC injection pipes 30 are provided at the cooling system of the PLWR to provide the emergency core cooling water in the PLWR, such as the DEGB in which a cold leg 15 is broken such that the cooling water flowing through the cold leg 15 does not reach the reactor core 7 of the reactor vessel 100, but is discharged to the outside of the cooling system of the PLWR through a double-ended broken area 35 of the broken cold leg 15, thus causing an increase of the cladding temperature of fuel rods to the highest point.
As an example of conventional ECC injection techniques, a cold leg injection technique in which the ECC is injected into the reactor vessel 100 through the cold legs 15 has been proposed. The PLWR, designed to use the cold leg injection (CLI) technique, is a so-called “cold leg injection-type PLWR (CLI-PLWR)”, and, in the CLI-PLWR, an ECC injection pipe 30 is connected to each cold leg 15, as shown in FIG. 1B. When the DEGB, in which a cold leg 15 is broken to form the double-ended broken area 35, occurs in the CLI-PLWR, all the fluid flowing in the broken cold leg 15 is discharged from the broken cold leg 15 to the outside of the cooling system of the PLWR through the broken area 35. Therefore, the ECC, injected into the broken cold leg 15 in the case of emergencies of the DEGB in the CLI-PLWR, does not cool the reactor core 7, but is lost through the broken area 35 of the broken cold leg 15. That is, the CLI-PLWR is problematic in that it undesirably causes ECC loss in the case of the DEGB.
In an effort to solve the above-mentioned problem of the CLI-PLWR, a direct vessel injection-type PLWR (DVI-PLWR), in which the ECC is directly injected into the downcomer 20 of the reactor vessel 100 as shown in FIGS. 2A and 2B, has been proposed. That is, the ECC in the DVI-PLWR is not indirectly injected into the reactor vessel 100 through the cold legs 15, but is directly injected into the downcomer 20 of the reactor vessel 100, so that the DVI-PLWR reduces the ECC loss even in the case of DEGB, different from the CLI-PLWR in which the ECC is lost through the double-ended broken area 35 of a broken cold leg 15 when DEGB occurs in the CLI-PLWR. The construction and operation of a conventional DVI-PLWR may be referred to Korean Patent Laid-open Publication No. 2001-76548.
However, the conventional DVI-PLWR is problematic in that a part of the ECC, directly injected into the downcomer 20 defined between the pressure vessel 5 and the core barrel 10 of the reactor vessel 100, undesirably flows to the broken area 35 of the broken cold leg 15 while being carried along in a high-speed lateral flow of steam which flows in the downcomer 20 and rushes from the downcomer 20 to the broken area 35 of the broken cold leg 15, so that the ratio of emergency core cooling water bypass (ECC bypass) is increased. In a detailed description, which is different from the conventional CLI-PLWR in which the low temperature ECC in the case of accidents in the PLWR is injected into the cold legs 15 to cause a large amount of steam condensation in the cold legs, the DVI-PLWR, in which the ECC is directly injected into the downcomer 20 of the reactor vessel 100, is less likely to cause the steam condensation in the cold legs. Therefore, the steam flow speed in the cold legs 15 of the DVI-PLWR is much higher than that of the CLI-PLWR.
Furthermore, in the reactor vessel 100 of the conventional DVI-PLWR, both the inner surface of the pressure vessel 5 and the outer surface of the core barrel 10 which define the downcomer 20 have a smooth surface, so that the ECC, directly injected into the downcomer 20, is easily separated from the smooth surfaces so as to flow from the downcomer 20 to the double-ended broken area 35 of the broken cold leg 15. That is, the ECC layer, formed on the smooth surfaces of the pressure vessel 5 and the core barrel 10, is easily separated from the smooth surfaces to form ECC drops and masses which are more easily carried along in the high-speed lateral flow of the steam to be bypassed to the broken area of the broken cold leg 15. Therefore, the ECC loss of the DVI-PLWR in the case of DEGB becomes larger.
Since the ECC, directly injected into the downcomer 20 of the reactor vessel 100 of the DVI-PLWR through the ECC injection pipes 30, flows easily from the downcomer 20 to the double-ended broken area 35 of the broken cold leg 15 in the case of DEGB of the PLWR while being carried along in the high-speed lateral flow of the steam, as described above, the ratio of the ECC bypass of the DVI-PLWR is undesirably increased.
The increase in the ratio of the ECC bypass of the DVI-PLWR is caused by the structural frailty of fluid flow in which the ECC layer is easily separated from the smooth surfaces of the pressure vessel 5 and the core barrel 10 to form ECC drops and masses, thus the ECC drops and masses are more easily carried along in the high-speed lateral flow of the steam so as to be bypassed to the broken area 35 of the broken cold leg 15. However no engineering technique to prevent the formation of the high-speed lateral flow of fluid including the ECC in the downcomer 20 in the case of the DEGB of the PLWR has been proposed in the prior art.
When the ratio of the ECC bypass is increased to exceed a predetermined level, the maximum cladding temperature of the fuel rods is increased, and the reactor core 7 is reheated at a late reflood phase after the LBLOCA, so that serious problems in the terms of the safety of the PLWR are caused. Therefore, in order to maintain the effectively cooled state of the reactor core 7, it is necessary to increase the capacity of an ECC pump installed at the ECC injection pipes 30 to enlarge the amount of the ECC, or provide a technical solution to reduce the ratio of ECC bypass. However, when the effectively cooled state of the reactor core 7 is accomplished by increasing the capacity of the ECC pump of the ECC injection pipes 30, instead of providing the technical solution to reduce the ratio of the ECC bypass, it results in an undesired increase in the production cost of the DVI-PLWR, due to the ECC pumps having a large capacity. This cuts down the economic advantages of the DVI-PLWR, which are expected from the reduction in the capacity of the ECC pumps followed by the change in the ECC injection technique from the CLI to the DVI technique.
As described above, the conventional DVI-PLWR is problematic in that it has a high ratio of ECC bypass, even though it has been proposed as a substitute for the conventional CLI-PLWR. Therefore, it is necessary to propose a technical solution to reduce ECC loss by reducing the ratio of ECC bypass, which is a phenomenon by which the ECC is undesirably bypassed from the downcomer 20 of the reactor vessel 100 to the double-ended broken area 35 of the broken cold leg 15 while being carried along in the high-speed lateral flow of the steam.